Overheating Ford Galaxy ?

I have had a Ford Galaxy for over 10 Years Now from New, and overall it has served me well. Until a few years ago, when I had a break in, and the electrics were damaged. It took 6 months and living with no car having to haggle with the Insurance company, who wouldnt pay up, cos they said I had used a Phone charger in the car and they classed that as a modification! Anyway, that cost me over £2,000 to put right, and then less than 6 months later, I started getting problems with Hill climbs, and long lourneys. the car struggled, so back it went in and the Ford garage claimed we needed a new clutch! So we paid out again over £1,300 this time. It still didnt solve the problem so they came up with another resolve Cant remember what now) but a further £300 was spent. It still happened, but not as often. Last week I went to Somerset and on the way back, the car started to overheat suddenly and the warning light came on.
We struggled to get home, by keeping the speed down and the heater in the car on! The next day we flushed the radiator through , toped up the car with antifreeze etc and booked it into the garage.
They gave it a full service £200 and new back brakes £200 and 3 new tyres £250.00! They couldnt recreate the over heating fault, and checked some obvious causes they said, but coudnt find anything wrong. We drove it home and it seemed fine for a few days, until I drove it to Devon and as soon as I had done a few hours driving the temp guage went into warning! I had to stop at Exeter, and wait for an hour before finishing my journey to Torquay. Getting home was a nightmare, as I had to keep to 40miles an hour, with heater on and windows open! Managed to get home safely though, and it seems OK around the town. Called the Ford Garage and they are convinced its the Water Tank, as they have seen this before. The water doesnt seem to be moving at all, in fact the water level rises! They have quoted over £600 to repair! They think this will cure it! I am not sure what to do! I dont have the money, and will have to spend my Bill money to pay for this. I am self employed so I need a car or I may as well pack Up now! Any thoughts???